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The homogeneity (concentration boundary of the precipitation of the MgO phase) in sol-gelderived microcrystals with a mixed ZnO-MgO-FeO composition has been investigated using the optical and X-ray diffraction methods. It has been shown that optical spectroscopy in this case is a more sensitive method. The luminescence line of a deep center at 700 nm has been observed for the first time and used as an indication of the precipitation of the MgO phase. Based on the experimental data, the concentration boundary of the precipitation of the MgO phase is evaluated to be 4–10 at %, which is in good agreement with the available theoretical estimate.

Abstract: The effect of copper doping on charge-orbital ordering in La1/3Ca2/3Mn1 − y Cu y O3 (0 ≤ y ≤ 0.07) is studied by measuring the temperature dependences of the magnetization, the electrical resistivity, and the heat capacity in combination with an electron microscopic investigation of the structure. It is demonstrated that copper doping leads to a lowering of the charge ordering temperature T CO and that this decrease is proportional to the decrease in the Mn3+ ion concentration. In the temperature range 5–300 K, the semiconducting pattern of the electrical resistivity persists for all values of 0 ≤ y ≤ 0.07. Electron microscope studies have shown that the presence of copper suppresses the formation of a regular superstructure, which is characteristic of the undoped starting compound, beginning already from low concentrations (y = 0.01). Differential scanning calorimetry revealed a substantial decrease in the transition entropy at the onset of charge ordering in copper-doped samples as compared to the starting compound. Doping with copper destroys long-range charge-orbital ordering and retains apparently only short-range order.

The electrical conductivity of La1 − xCaxMnO3 − α(x= 0.5) alloys in a magnetic field of 0.6 T is investigated in the temperature range 78–300 K. According to the phase diagram, this compound at α = 0 corresponds to an intermediate region between the ferromagnetic conducting (x< 0.5) and antiferromagnetic semiconducting (xs> 0.5) phases and contains islands of the metallic phase. Under oxygen-deficient conditions, the fraction of the metallic phase increases and the semiconductor-metal transition takes place. The formation of bound islands of the metallic phase is observed under oxygen-deficient conditions in the La0.4Ca0.6MnO3 − αcompounds.

Abstract: The temperature dependence of the electrical resistivity ρ(T) for ceramic samples of LaMnO3 + δ (δ = 0.100–0.154) are studied in the temperature range T = 15–350 K, in magnetic fields of 0–10 T, and under hydrostatic pressures P of up to 11 kbar. It is shown that, above the ferromagnet-paramagnet transition temperature of LaMnO3 + δ, the dependence ρ(T) of this compound obeys the Shklovskii-Efros variable-range hopping conduction: ρ(T) = ρ0(T)exp[(T 0/T)1/2], where ρ0(T) = AT 9/2 (A is a constant). The density of localized states g(ɛ) near the Fermi level is found to have a Coulomb gap Δ and a rigid gap γ(T). The Coulomb gap Δ assumes values of 0.43, 0.46, and 0.48 eV, and the rigid gap satisfies the relationship γ(T) ≈ γ(T v)(T/T v)1/2, where T v is the temperature of the onset of variable-range hopping conduction and γ(T v) = 0.13, 0.16, and 0.17 eV for δ = 0.100, 0.125, and 0.154, respectively. The carrier localization lengths a = 1.7, 1.4, and 1.2 are determined for the same values of δ. The effect of hydrostatic pressure on the variable-range hopping conduction in LaMnO3 + δ with δ = 0.154 is analyzed, and the dependences Δ(P) and γv(P) are obtained.

Abstract: The structural, electrical, and magnetic properties of ceramic perovskite manganites LaMnO3 + δ (δ = 0–0.154) are investigated. It is found that, in a weak magnetic field (B = 2 G), the LaMnO3 + δ manganite with δ = 0.065 at temperatures below the Curie temperature T C of the paramagnet-ferromagnet phase transition has a mixed (spin glass + ferromagnet) phase. In LaMnO3 + δ manganites with the parameter δ = 0.100–0.154, this phase transforms into a frustrated ferromagnetic phase. A similar transformation was observed previously in La1−x CaxMnO3 compounds at calcium contents in the range 0 ≤ x ≤ 0.3. This similarity is explained by the fact that, in both materials, the Mn4+ concentration and, accordingly, the hole concentration c change equally in the concentration range from ∼0.13 to 0.34 with an increase in x or δ. However, the magnetic irreversibility, the concentration dependences of the Curie temperature T C(c) and the magnetic susceptibility X(c), and the critical behavior of the temperature dependence of the susceptibility X(T) in the vicinity of the Curie temperature T C differ substantially for these two materials. The observed differences are associated with the distortion of the cubic perovskite structure, the decrease in the degree of lattice disorder, and a more uniform distribution of holes in the LaMnO3 + δ manganites as compared to the La1 − x CaxMnO3 compounds.

Abstract: The formation and specific features of the superstructure in La0.33Ca0.67Mn1−y Fe y O3 (y = 0, 0.05) manganites doped with iron are investigated using transmission electron microscopy. The electron diffraction patterns of the manganites are studied in the temperature range 90–300 K, and the high-resolution electron microscope images recorded at temperatures of 91–92 K are analyzed. In both manganites, the structural transition that is accompanied by the formation of the superstructure and which is directly observed from the appearance of additional peaks in the electron diffraction patterns occurs at a temperature that is in close agreement with the charge ordering temperature T CO determined from the temperature dependences of the magnetization M(T). In the temperature range 90 < T < 200 K, the undoped compound has a commensurate superstructure characterized by the vector q = 1/3a* and triple the unit cell «3a × b × c» (where a ≈ b ≈ √2a c , c ≈ 2a c , and a c ∼ 3.9 Å is the lattice parameter of a simple perovskite). The doping with iron (5 at. %) brings about a decrease in the charge ordering temperature T CO by 50 K and the formation of an incommensurate structure for which the magnitude of the vector q is smaller by approximately 15%. The unit cell of the superstructure in the iron-doped compound is not triple the unit cell but involves defects of ordering, such as quadrupling of the unit cell, numerous translations by a c √2 along the a direction, and dislocation-type defects in the stripe structure of the charge ordering. These pseudoperiodic defects lead to a decrease in the magnitude of the vector q and are responsible for the incommensurability of the structure. A decrease in the charge ordering temperature T CO due to the doping with iron and the incommensurability of the superstructure correlate with the change in the concentration of Mn3+ Jahn-Teller ions as a result of their replacement by Fe3+ non-Jahn-Teller ions.

YBCO films were prepared by pulsed laser deposition from nanocrystalline targets doped with different concentrations of BaZrO3 ranging from c = 0.9 to 9.0wt%. The critical temperature of the films decreases almost linearly with increasing BaZrO3 content whereas the critical current density shows a maximum near 3.9wt%. In comparison with undoped YBCO films the accommodation field B* is considerably enhanced and the critical current density is improved in high fields in the films doped with BaZrO3, e.g. for c = 3.9 wt% by factor of 4.5 in a field of 5T at 5K. In the doped films the BaZrO3 particles grow epiaxially with YBCO. Transmission electron microscopy results show that the density of the BaZrO3 particles increases with increasing doping but the particle size remains practically the same (5–10nm).

YBCO films were prepared by pulsed laser deposition from nanocrystalline targets doped with different concentrations of BaZrO3ranging from c= 0.9 to 9.0 wt%. The critical temperature of the films decreases almost linearly with increasing BaZrO3content whereas the critical current density shows a maximum near 3.9 wt%. In comparison with undoped YBCO films the accommodation field B*is considerably enhanced and the critical current density is improved in high fields in the films doped with BaZrO3, e.g. for c= 3.9 wt% by factor of 4.5 in a field of 5 T at 5 K. In the doped films the BaZrO3particles grow epiaxially with YBCO. Transmission electron microscopy results show that the density of the BaZrO3particles increases with increasing doping but the particle size remains practically the same (5-10 nm).

The effect of annealing temperature for oxygenation (Toxg) on grain boundary (GB) segregation and the distribution of calcium in the bulk was studied for Dy123/Caz(z= 0, 0.2) ceramics for Toxg= 480 and 550?°C, where the first case corresponds to full oxygenation and the second to a little oxygen deficiency. Energy selective x-ray analysis techniques (with an ultra-fine probe) in field emission transmission electron microscopy were used for the characterization of the microstructure. It was found that Ca segregates at GBs in both cases, but the extent of such segregation strongly depends on Toxg. The GB segregation for Toxg= 550?°C was much lower than for 480?°C. For Toxg= 480?°C, the calcium concentration was 1.5-3.5 times higher than in the bulk in 60% of the GBs, and segregation was absent in 40% of the GBs. For Toxg= 550?°C, only 25% of the GBs showed Ca segregation, with a concentration 1.2-1.3 times exceeding that in the bulk. In both cases, the width of the segregation profile was about 10 nm. The change in Toxgalso affected the distribution of Ca in the bulk. In this way, the changes in the superconducting properties were directly correlated to the microstructure.

Abstract: Doping the YBaCuO superconducting ceramic with 3–5 wt % silver is shown to substantially increase its microplasticity and strength. Temperature spectra of microplastic strain rates for YBaCuO/Ag ceramics with 0, 1.6, 2.9, 4.2, and 7.6 wt % silver were obtained. The structure of the spectra and the relation of one of the peaks to the superconducting transition are discussed. The specific features of the I–V characteristics of samples cooled in a magnetic field are considered. It is shown that the position of the rising branch of a I–V characteristic depends on its origin; this observation can be used to fix two or more stable states of an HTSC-based memory cell.

Abstract: This paper reports on a first study of the effect of oxygen content in La0.67Ca0.33MnO3−α on the electrical conductivity, magnetoresistance, and specific heat within a broad range of oxygen deficiency variation, 0≤α<0.4. Preparation of a large quantity of stable samples with an easily controllable oxygen deficiency was made possible by using a “soft” hydrogen treatment (870 K, 1–2 kPa). The properties of samples with 0≤α≤0.06 are shown to differ from those with α≥0.1. For instance, samples studied in the semiconducting phase at 300 K revealed, on the average, E g=0.28 eV in the first group and E g=0.43 eV in the second, and only samples of the first group exhibit giant negative magnetoresistance and specific-heat anomalies in the range 150–200 K. The latter anomalies correlate with magnetoresistance peaks observed in all samples of the first group (with α≤0.06). The metal-semiconductor transition was found to disappear abruptly for α>0.006. The effects of a variation in the cation composition and the oxygen content on the physical properties are compared.

1.The nonuniform effect of average defect density and average stress level on the corrosion resistance of steel is due to evolution of a substructure and connected with it the nonuniform dependence of local internal stresses.2.A new test procedure for corrosion-mechanical strength provides quantitative estimates of the test results.3.Hot-working may be used in order to prepare articles intended for operation in corrosive media with high stresses.

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Abstract: The electrical conductivity of ceramic samples with a general formula HxBa2YCu3O7+z has been measured in the course of hydrogenation (formation of the hydrocuprate) and subsequent oxidation (formation of the oxyhydrocuprate). The hydrocuprate (x>0, −0.12YCu3O6.9 ceramic to hydrogen at a temperature of 450 K and a pressure of 50 kPa. The oxyhydrocuprate (x=2, z⩽1.5) was prepared by treating the preliminarily hydrogenated ceramic of composition H2Ba2YCu3O6.9 with oxygen at 470 K at a pressure of 70 kPa. The four-probe dc method was used in the electrophysical measurements. The measurements were performed in the temperature interval 300–450 K and within the compositional ranges 0

DSC investigations have been performed for a series of compounds Ba2YCu3Oy with the oxygen content varying in the rangey=6.0...6.9 by means of various heat treatments at 800–1200 K followed by quenching, or through the chemical extraction of oxygen by placing the sample in dihydrogen at 470–490 K. The sample preserving a constant oxygen content during heating in nitrogen exhibited exothermal effects between 450 and 850 K. It has been shown that the ΔH vs. y function reaches maximum aty ≈ 6.5. Kinetic measurements have shown that the diffusive mobility of oxygen atoms in the lattice is responsible for these effects, viz. the Arrhenius and cooperative processes of reorganization in the non-equilibrium oxygen subsystem of the bulk.

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1.A fundamentally new hypothesis is suggested for occurrence of self-ignition in coal.2.The hypothesis obtained is checked by calculations for a linear unit and a structural unit of coal.3.The probability of forming seats of self-ignition at a distance from the combustion front is higher than close to it.4.Conditions for forming seats of self-ignition do not require transfer of coal into a plastic state and application of a possible hypothesis of cumulative collapse of gasfilled bubbles with release of the required energy (apart from the case of µ2=402).5.On the basis of the hypothesis suggested it is possible to suggest a mechanism of forced intensification of coal seam combustion by converting it into coal detonation.

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Recently created pulsed gas lasers with high pulse repetition rate produce radiation in the IR [ V. Yu. Baranov , in Molecular Gas Lasers. Physics Applications, VelikhovE. P., Ed. ( Mir, Moscow, 1981), pp. 252– 255] and the UV ( V. Yu. Baranov , in Proceedings, International Conference on Lasers, 14–18 Dec. 1981, pp. 968– 974) regions of the spectrum. These lasers are widely used in research on the selective excitation of matter. This field of study includes laser isotope separation { G. I. Abdushelashvili , Kvantovaya Elektron. (Moscow)9, 743 ( 1982) [ Sov. J. Quantum Electron.12, 459 ( 1982)]} and laser-induced chemical reactions [ V. Yu. Baranov , Preprint IAE 3693/13 ( Moscow, 1982)]. But there are few publications dealing with uses of high-repetition-rate lasers for materials processing, although this problem undoubtedly is of considerable scientific interest.

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The velocity and size distribution of high-speed glass microparticles in a flow under idenical conditions of acceleration is estimated with the use of the known statistical approach. The results are applied for the study of the interaction of microparticles with a system of polymer obstructions made of polyethylene and polyethelene terephthalate. The critical impact velocities corresponding to complete transition of the particle to the viscous-fluid state are determined.

The authors present the results of experiments to determine the dependence of the coefficient ?, which takes into account the nonstationarity of the impact process, on the impact velocity and the properties of the target and projectile material. Data are obtained on the depth of penetration of low-melting modeling materials into semi-infinite targets; these data are then used to verify previously proposed modeling parameters.

Abstract: A study has been made of the magnetic and electrical properties of oxygen-enriched La1 − x Ag x MnO3.1 manganites at 77 K < T < 300 K. The samples have been produced by the sol-gel technique. The study has revealed an increase of the Curie temperature and a shift of the maximum in magnetoresistance toward high temperatures.

Abstract: The electrical conductivity, magnetization, and magnetoresistance of manganites La1 − x Ag x MnO3 have been investigated in the temperature range 78–300 K. The samples have been synthesized by the sol-gel method. At room temperature, the magnetic field of 0.6 T has no effect on the electrical conductivity. As the temperature decreases, an abrupt jump is observed in the magnetization curve due to the semiconductor-metal phase transition. This transition hardly affects the temperature dependence of the resistance.

An expression describing the shrinkage rate of compacts composed of nanoparticles is obtained under the assumption that nanoparticles undergo fluctuation melting. The expression derived has the following form: dY/dt= Aexp(−(TM+ q)/T), where dY/dtis the shrinkage rate; TMis the melting temperature of nanoparticles; Tis the sintering temperature; and Aand qare constants that depend on the material, the initial porosity, and the size of nanoparticles in the compact. The experimental data on the shrinkage rate of compacts composed of copper and nickel nanoparticles 70 nm in size are compared with the shrinkage rates calculated according to the derived expression. This comparison shows that the experimentally measured shrinkage rates are close to the calculated values.

Abstract: Thallium halide microcrystals were grown from water solutions in a porous matrix obtained from alkali borosilicate glass by etching, and their absorption and luminescence spectra were studied. Constraining the crystal size was found to affect the stability of some crystal modifications. The exciton radii were determined, and the dimensions of microcrystals in which size quantization effects become observable were estimated from the absorption spectra of bulk TlBr and TlCl crystals.

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1.Titanium alloys with 9% Al and 3% Zr and varying amounts of tungsten (0–10%) undergo brittle fracture in the forged condition (without heat treatment) at 20°.2.After annealing at 900° for 1 h and stepped annealing at 900° for 20 h+800° for 100 h+600° for 100 h the ductility of the alloys at 20° does not increase, but at 600° the alloys become ductile.3.In the range of the highest solubility of tungsten in a titanium (~0.7% W) a slight peak of hardness and ultimate tensile strength, and lowest ductility, is visible on the curves at 600°. With further increase of the tungsten content the hardness and ultimate strength increase monotonically, almost linearly, and the ductility decreases.

We determine the time of the loss of dynamic strength for an arbitrary rate of strain from an external load source until reaching overstresses on the stressed discontinuities that are equal to the value of the critical stress, which is a property of the material and of its characteristic dimension. We obtain the quadrature of the basic equation of overstress accumulation with the aid of the time dependence of the elastic modulus E. We find that numerical integration of this equation with the found experimental relationship E(t) (t is time) is not possible because of the presence of singularities on the interval of time variation, in which a solution does not exist.

The results of experiments to determine some of the parameters characterizing the perforation of a barrier made of a series of materials are set forth. These include the momentum imparted to the barrier when it is perforated, the diameters of the holes, the angles of front and rear dispersion, and a number of others.

Preparation rotes of YBCO nanopowders based on a citrate-gel method and calcination after reduction of oxygen from the precursor powder are discussed. High-quality thin films have been deposited by a XeCl laser using targets pressed from these powders.

A model of vortex trapping and relaxation in particles with sizeD<λ (λ is the magnetic field penetration depth) distinguished from critical state model by accounting size-dependent relaxation rate, threshold field and current is considered. Experimentally observed strong suppression of remanent magnetization in submicron YBCO particles is in accordance with this model.